Modular high power solid state amplifier

The present invention relates generally to amplifiers and, in particular, to a modular high power solid state amplifier.

Power outputs from individual amplifiers or amplifier devices are often combined to achieve a higher output power. The power output from one amplifier is combined with the power output from another amplifier and so on until the desired level of output power is reached.

There is some loss associated with combining multiple amplifiers due to the splitting and combining circuitry. This loss should be considered when determining the desired output power.

There are power limit constraints on solid state amplifiers. For example, due to combiner losses and device output power, a low cost, conventional solid state power amplifier can only achieve a few watts. For high power applications (e.g., over 10 watts), typically only tube amplifiers are available and not solid state devices. Moreover, even if solid state electronics were available for high power amplifiers, current assembly techniques require the size of the power amplifier to be too large and costly.

There are numerous problems associated with current power combining methods. Power combining requires the phase and amplitude of the combined signals to be closely matched. High frequency power amplifiers typically require precise, labor-intensive hand tuning to set the phase for each individual amplifier, thereby increasing labor and/or testing costs. Moreover, hand tuning the power amplifier increases manufacturing difficulty.

The assembly of each power amplifier is often complicated and time consuming. For example, RF (radio frequency) power amplifiers typically require a DC (direct current) feed to each individual amplifier. The DC must be routed around the RF paths channeling RF to each individual amplifier on a RF board. In the past, small wires (jumper wires) have been used to “jump over” the RF paths and deliver the DC to the amplifiers. Individually bonding the DC wires to the RF board is time consuming. Additionally, the fragile wires are very thin and susceptible to breakage; thus increasing the risk of amplifier breakdown. Moreover, the DC component is not integrated with the power amplifier into a single stand-alone package, but rather, the DC component is built into the RF component.

The various aspects of the present invention address the problems outlined above and provide a method and apparatus for an improved high power amplifier. A modular solid state high power amplifier is herein disclosed having a DC component and a RF component coupled to at least one gain device, and a suitable signal isolation layer between the two components.

The solid state components, angled configurations, and piggyback topology of the invention provide a compact and efficient integrated amplifier device and method of assembly, manufacture, and use.

These and other features, aspects and advantages of the present invention will become better understood with reference to the following description, appending claims, and accompanying drawings where:

FIG. 1 illustrates a modular power amplifier in accordance with one embodiment of the invention;

FIGS. 2 and 2 a - 2 c illustrate the RF portion of a power amplifier in accordance with one embodiment of the invention; and

FIG. 3 illustrates a block diagram of a power amplifier in accordance with the invention.

Various embodiments of the present invention relate to an improved method of assembly for a solid state power amplifier. The power amplifier structure is particularly suited for use in high power applications in connection with MMIC (monolithic microwave integrated circuit) technology. As a result, the exemplary embodiments of the present invention are conveniently described in that context. It should be recognized however, that such description and drawings are not intended as a limitation on the use or applicability of the present invention, but rather, are provided merely to discuss one possible application. Accordingly, various other amplifiers may be equally suited for use in conjunction with the invention for a wide array of applications and systems, e.g., test amplifiers, customer premises equipment for LMDS, and satellite systems.

Although the high power amplifier disclosed herein may be suitable for mobile communication in a variety of applications, the present invention may be conveniently described with reference to commercial satellites systems or terrestrial point to multi-point communication systems, and more particularly to ground based Ka-band transmitters operating between about 26.5 GHz and 32 GHz.

An improved high power solid state amplifier and method of assembly is herein provided. For example, operating at a frequency of about 27 to 30 GHz, a single (integrated) module power amplifier in accordance with the invention can achieve a range of power outputs between about 10 to 14 watts. The solid state components, angled designs, and piggyback topology of the invention provide a small, highly efficient, integrated amplifier device.

FIG. 1 illustrates a power amplifier module 100 in accordance with one embodiment of the invention. Power amplifier 100 includes a RF board 120 , a plurality of amplifiers 150 , an RF cover 130 having an interlocking input 135 , and a DC board 140 . Additionally, power amplifier 100 may be a fully integrated structure having a top cover 170 , a chassis 110 , a plurality of driver amplifiers 155 , and a plurality of pedestals 160 .

DC board 140 preferably includes a rigid non-conducting, multi-layered dielectric substrate material including, but not limited to, fiberglass and polyimide. DC board 140 suitably includes trace lines to route DC to one or more bias points. Each bias point includes a connector plug-in point. For example, one end of a connector plug-in is suitably located on RF board 120 at a desired bias point and the mated end of the connector plug-in is located on DC board 140 such that when the two plug-in ends are brought together, a DC connection is made for routing DC to RF board 120 . The size of DC board 140 varies depending upon the number of bias points on RF board 120 .

DC board 140 promotes fast self-alignment of the connection points for easy “plug-in” and may also include any number of flexible interconnects 145 . Flexible interconnect 145 allows movement between two rigid portions of DC board 140 . This is particularly useful when matching (connecting) several bias points. DC board 140 , in particular the plug-in design, facilitates fast assembly, disassembly, and maintenance of the power amplifier module.

The improved assembly of the power amplifier of the present invention permits the DC board to be integrated into a single module with the RF board. RF cover 130 provides suitable signal isolation between RF board 120 and DC board 140 . The DC can be routed over the top of the RF in, for example, a daughter board configuration. This configuration eliminates the need for jumper wires extending from the DC board to the RF board. In addition, the “piggyback” power distribution topology (i.e., one board on top of the other) further reduces the overall size of the power amplifier. In one particular embodiment of the invention, the stand-alone power amplifier unit has dimensions on the order of 139.7×127×25.4 mm. Moreover, the DC board can be tested separately from the RF board. Thus, it can be determined prior to assembly if the board is in working condition. This results in a higher overall yield for the power amplifier module.

The DC board may further include a regulator and a switch (not shown). A single DC voltage is received at the DC board. The regulator conditions the DC voltage and controls the distribution of voltage to each individual component. The amount of bias is determined by the components on the RF board. Each individual component may require a different level of bias and the regulator suitably distributes the proper level.

An electronic switch turns the power amplifier on and off based on a signal received at the input connector. Generally, the switch, nearly simultaneously, turns each individual amplifier on and off at the same time as every other amplifier.

In one embodiment, DC board 140 is secured to the module via one or more plug-in connectors located on RF board 120 and one or more fixation devices. For example, fixation devices, such as screws, may attach DC board 140 to chassis 110 . Securing the DC board helps to maintain the plug-in connectors positions. Additionally, securing the DC board to the chassis helps to ensure DC grounding.

RF cover 130 includes a metal material that suitably provides signal isolation between the DC and RF lines. RF cover 130 further provides a seal for many of the cavities of RF board 120 . A conductive epoxy-like substance, such as silver epoxy, may be used to suitably bond RF cover 130 to the chassis. Preferably, the bond area promotes high RF conductivity, environmental protection (i.e., substantially free from dust and contaminants), and provides an EMI (electromagnetic interference) seal.

RF cover 130 includes an interlocking input 135 preferably made from the same material as cover 130 . Interlocking input 135 suitably includes a waveguide for module RF input.

Interlocking input 135 is completely detachable from cover 130 . RF cover 130 may be aligned atop RF board 120 and the waveguide port in chassis 110 , and in particular, the RF output port and RF input port can be aligned separately. The two ports of RF cover 130 are substantially aligned with the corresponding ports on RF board 120 and the ports in chassis 110 . Maintaining port-to-port alignment (i.e., RF input port on RF board 120 and in chassis 110 to RF input port on cover 130 ; RF output port on RF board 120 and chassis 110 to RF output port on cover 130 ) reduces RF loss. Each port-to-port alignment may be done separately to increase accuracy. Once the output ports are aligned, the input ports can be aligned using the moveable interlocking input 135 .

In one embodiment, interlocking input 135 and the corresponding “locking” area of cover 130 are configured in a “tongue-and-groove” manner. For example, interlocking input 135 may suitably include a groove edge such that input 135 can easily be slipped over the matching tongue portion of cover 130 . Input 135 can be moved along the tongue portion for aligning purposes. The input ports are aligned and the cover, including the interlocking input portion, is bonded in place.

RF board 120 includes a low loss substrate suitable for RF applications including, but not limited to, many teflon-based materials. RF board 120 suitably includes trace lines for routing RF amplifiers 150 , 155 . Amplifiers 150 , 155 are electrically connected to RF board 120 by, for example, wire bonding each amplifier to the trace lines.

In order to provide a good RF ground connection, RF board 120 may be epoxied to the chassis using an epoxy-like substance.

Amplifiers 150 may include any amplifier of preferably high gain and efficiency (e.g., DC to RF conversion). Preferably amplifiers 150 include the same or substantially the same type of amplifiers, as well as the same power output. MMIC amplifiers are particularly suited for power amplifier applications. Typically, MMICs are available as a pre-packaged device having a protective cover; thereby making handling of the MMIC easier during power amplifier assembly. While MMICs are conveniently described, various other amplifiers may be suited, such as amplifiers made from discrete FETs.

The number of amplifiers 150 is generally chosen as a power of 2 (i.e., 2 N ) due to the power combining network. Four (2 2 ) amplifiers 150 are illustrated in FIG. 1, however, the number of amplifiers, e.g., eight (2 3 ), sixteen (2 4 ), and so on, may be modified as needed to obtain the desired power output. While 2 Nth number of amplifiers is herein disclosed, the number and combinations may be varied for each particular application.

Top cover 170 includes a suitable metal material. Top cover 170 is particularly useful in isolating radiating signals, providing structural strength to the integrated module, and protecting the underlying components.

In one embodiment, top cover 170 is fitted by one or more posts extending from cover 170 . The bottom outer edge of top cover 170 is bonded to a top outer edge of chassis 170 with a suitably epoxy-like substance; thereby encasing the inner layers to form an integrated substantially stand-alone power amplifier.

Top cover 170 may further include pilot holes 180 on the outer edge for inserting a fixation device, such as a screw, to be received in matching pilot holes 185 located on the outer edge of chassis 110 .

Chassis 110 includes a metal material that is preferably easy to fabricate, provides grounding, thermal conductivity, mechanical support and integrity (e.g., aluminum). Holes are suitably drilled in chassis 110 in the areas where amplifiers 150 are to be mounted. Other areas may be suitably removed from chassis 110 that correspond to overlying components and boards, such as RF board 120 .

Driver amplifiers 155 include any suitable gain devices typically coupled in series. In general, the number of individual amplifiers varies depending upon the desired gain of the power amplifier module. The signal input at the module can be a low power signal and the driver amplifiers suitably provide enough gain (i.e., increase (amplify) the signal) to the output amplifiers 150 to overcome any loss due to the splitter circuitry.

Pedestals 160 are included as needed to protect the amplifiers from mechanical overstress resulting from, for example, the mismatch in thermal expansion coefficient between the amplifier and the material of the chassis (e.g., aluminum). In cases where the material of the chassis matches or substantially matches the material of the amplifier, the need for a pedestal diminishes. Materials such as beryllium oxide, copper tungsten, and copper moly tend to match or substantially match the amplifier and may be used for the chassis. However, as available currently, these materials are expensive, heavy, and more difficult to machine than aluminum. Additionally, where the amplifier is attached with an epoxy-like, pedestals are generally not needed.

Each amplifier 150 , 155 may be suitably bonded atop a pedestal 160 . For example, the amplifier may be attached with an eutectic solder or some other method having low expansion modulus. For example, eutectic gold tin solder may be used to attach the amplifier, especially when high thermal and electrical conductivity are desired (e.g., amplifier 150 ). Driver amplifiers 155 tend to dissipate less energy and therefore may be attached with a suitable epoxy or epoxy-like substance and thus a pedestal may not be required.

In the case where pedestals are used, each pedestal 160 , having an amplifier atop, may be fitted into a cutout portion of chassis 110 . The pedestals may be bonded to the chassis using any suitable bonding material, such as an epoxy-like substance or eutectic solder.

Power amplifier 100 may further include a heat sink (not shown) of suitable material for providing thermal coupling. In one embodiment, a heat sink may be mounted to the bottom of chassis 110 . The fixation device(s) inserted into pilot holes 180 , 185 may attach the heat sink to power amplifier 100 .

FIGS. 2 and 2 a - 2 c illustrate a top view of a lower portion of an exemplary power amplifier structure 200 in accordance with the invention. Structure 200 includes an input 202 , an output 204 , a chassis 210 , a RF board 220 , a plurality of suspended strip line combiners 230 , a plurality of loads 240 , a plurality of output amplifiers 250 , one or more driver amplifiers 255 - 257 , a plurality of isolation components 260 and 265 , and one or more DC connectors 270 .

Typically, each output amplifier 250 is coupled to power splitter and combiner circuitry by, for example, etched transmission line technology. Etched transmission lines help to minimize phase variations between the amplifiers. Power loss associated with this circuitry is a function of the travel length (i.e., line length), thus it is advantageous to reduce the line length used for splitting and combining. The power level at the input (splitter circuitry) is low, so while reducing the input line length does help to decrease power loss, the power savings tends to be trivial. On the other hand, after the power gain from each of the amplifiers is added together and the DC power is invested, the output loss associated with the length of the combiner line can be significant. Thus, a small improvement in the amount of power loss associated with the output (combiner circuitry) can increase the overall efficiency of the power amplifier.

In the present embodiment, amplifiers 250 are suitably configured in an angled formation. By angling the amplifiers, the length of the lines, and in particular the output line, are considerably shorter. Reducing line lengths on the RF board helps to reduce the overall size of the power amplifier module, decrease output power losses, and increase overall system efficiency.

Another advantage of angling amplifiers 250 in the present configuration, is that each amplifier receives substantially the same level of airflow. Air typically flows orthogonal to either side of amplifier 100 . Amplifiers 150 and 155 are configured such that a substantially even distribution of amplifiers over the area of the chassis results. This configuration helps to reduce “hot spots” (created when heat released from nearby devices is allowed to combine) and increase amplifier performance and reliability.

In one embodiment, system 200 includes four substantially identical 4 watt amplifiers 250 , such as MMIC amplifiers. At an operating frequency of about 27 GHz, the power from each of the four MMIC amplifiers can be combined to obtain a high power output of about 14 watts. It should be appreciated that various other types and power output amplifiers may be implemented; operating frequencies may vary depending upon the application; and total power output of the power amplifier may vary depending upon the desired output, operating frequency, number and type of amplifier(s) used, and operating conditions. Additionally, the modular solid state power amplifier herein disclosed may be coupled with one or more similar power amplifiers; thereby resulting in a combined modular power amplifier system having an increased total power output.

Each driver amplifier 255 represents a gain block, where each gain block couples to another gain block in series. The first two driver amplifiers 255 , 256 may be low power amplifiers. The final driver amplifier 257 may be a high power amplifier (e.g., the same type of amplifier as the output amplifiers 250 ) capable of supplying enough power to drive the four amplifiers 250 . RF input 202 may be a low power input suitably received at first driver power amplifier 255 . In the present embodiment, three drivers provide the desired power gain to drive amplifiers 250 . However, it should be realized that the number of driver amplifiers and the amount of power output varies depending upon the desired gain of the module.

In the present embodiment, structure 200 includes suspended strip line combiners 230 in a quadrature configuration. Suspended stripline combiners 230 provide improved RF isolation and power handling capability between individual amplifiers 250 , and reduced insertion loss and phase variations.

Typically in quadrature coupling configurations, one of the four legs includes a load to provide isolation between the two combining ports. The quality of the load is a fundamental part of the isolation. Any unbalanced signal gets suitably shunted over to the isolation port where the load is preferably matched to reduce reflection. During normal operation of the power amplifier, if one of the legs become inoperative, roughly half of the power from the defective leg can be rerouted to the isolation leg and the system can still function (although usually at a reduced performance).

Loads 240 of the present embodiment, may be comprised from a ferrite filled silicone rubber, or other suitable material having a high dielectric constant. Loads 240 are configured as suspended stripline loads designed to handle high power with reduced reflection loss.

In one embodiment, loads 240 are angled in two planes. In contrast, a conventional load includes a rectangular shape having four straight lined surfaces. An undesirable rapid impedance shift can occur, especially in high dielectric constant materials when the signal reaches a rectangular-shaped load. The angled load configuration of the present invention permits a gradual transition of the signal to the load, thereby effectively reducing the impedance shift.

In one aspect of the present embodiment, loads 240 are in a suspended stripline configuration which further helps to reduce the impedance shift. In addition, the suspended stripline configuration helps to increase attenuation per unit length.

As previously mentioned, loads 240 may comprise a ferrite filled silicone rubber. As those skilled in the art will recognize, a similar type material may be used to provide RF isolation around the DC bias injection points and mode suppression in the large cavities.

Typically in high current applications, isolation between the amplifier stages is desirable to prevent loop oscillations. System 200 includes DC isolation components 260 (capacitors) and 265 (ferrite chips). Capacitors 260 are suitably configured as bypass (shunt) capacitors and are mounted near amplifiers 250 . Ferrite chips 265 (or similarly acting resistance) are configured for series isolation and are also suitably mounted near amplifiers 250 . The number of capacitors and total capacitance, as well as the number of ferrite chips and resistance, varies depending upon the particular matching needs of each application.

DC connectors 270 suitably include the RF end of the DC connector plug-in. Each DC connector 270 may be considered a “bias point” however, as illustrated, the DC is suitably routed from the connector point 270 to the bias point(s) on each amplifier 250 , 255 . As previously disclosed, DC connectors 270 make up one end of a connector plug-in with the other plug-in end located on a DC board (e.g., DC board 140 ). The two plug-in ends are brought together and a DC connection is made for routing DC to RF board 120 . Each DC connector 270 may further include various other components to aid in the receipt and distribution of the DC (e.g., bias resistors, routing circuitry, and the like).

As previously mentioned, a modular solid state power amplifier in accordance with the invention includes power splitter and combiner circuitry. With continued reference to FIG. 2, the various pathways of the splitter circuitry and the combiner circuitry are exemplified. It should be appreciated that FIG. 2 is merely one representation of a variety of possible circuitry layouts for routing the splitter and combiner circuitry.

Referring now to FIG. 3, a block diagram of a power amplifier system 300 in accordance with one embodiment of the invention is illustrated. System 300 includes driver amplifiers 310 - 312 , splitter circuitry 320 - 322 , output amplifiers 331 - 334 , and combiner circuitry 340 - 342 .

Driver amplifier 310 receives, amplifies, and then passes the RF input signal, which is typically a low power signal, to driver amplifier 311 . Driver amplifier 311 also amplifies the signal and passes the signal to driver amplifier 312 . Driver amplifier 312 further amplifies the signal to an appropriate gain level such that the amplified signal can now be divided into enough equal inputs to drive the output amplifiers. These steps may be carried out more or less times than three, depending on the amount of input signal gain desired.

Splitter circuitry 320 receives the amplified signal and suitably divides the signal into substantially equal inputs to be received at splitter circuitry 321 , 322 . Splitter circuitry 321 and 322 may be quadrature configurations, thereby providing signals to output amplifiers 331 - 332 and 333 - 334 respectively, at a 90° phase shift. For example, amplifier 331 receives substantially the same input signal as amplifier 332 , however the signal is phase shifted 90°. Amplifier 333 receives substantially the same input signal as amplifier 334 , however the signal is phase shifted 90°. In addition, amplifier pair 331 , 332 is phase shifted 90° from amplifier pair 333 , 334 .

Output amplifiers 331 - 334 are preferably all the same type and power output. Each output amplifier provides a high gain (high output power) that is combined with the other amplifiers gain by the combiner circuitry.

Combiner 340 receives the combined output power from amplifier pair 331 , 332 (via combiner 341 ) and the combined output power from amplifier pair 333 , 334 (via combiner 342 ). Combiners 340 , 341 , 342 combine the power from the amplifiers in phase. Thus, at output combiner 340 , the high power output is in phase.

It should be appreciated that the particular implementations shown and described herein are illustrative of various embodiments of the invention including its best mode, and are not intended to limit the scope of the present invention in any way. Indeed, for the sake of brevity, conventional techniques for signal processing, data transmission, signaling, and network control, and other functional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical communication system.

The present invention has been described above with reference to exemplary embodiments. However, those skilled in the art having read this disclosure will recognize that changes and modifications may be made to the embodiments without departing from the scope of the present invention. For example, various types of amplifiers may be included. These and other changes or modifications are intended to be included within the scope of the present invention, as expressed in the following claims.